This invention relates to methods for driving electro-optic displays, especially bistable electro-optic displays, and to apparatus for use in such methods. More specifically, this invention relates to driving methods and apparatus (controllers) which are intended to enable more accurate control of gray states of the pixels of an electro-optic display. This invention also relates to a method which enables long-term direct current (DC) balancing of the driving impulses applied to an electrophoretic display. This invention is especially, but not exclusively, intended for use with particle-based electrophoretic displays in which one or more types of electrically charged particles are suspended in a liquid and are moved through the liquid under the influence of an electric field to change the appearance of the display.
In one aspect, this invention relates to apparatus which enables electro-optic media which are sensitive to the polarity of the applied field to be driven using circuitry intended for driving liquid crystal displays, in which the liquid crystal material is not sensitive to polarity.
The term “electro-optic” as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.
The term “gray state” is used herein in its conventional meaning in the imaging art to refer to a state intermediate two extreme optical states of a pixel, and does not necessarily imply a black-white transition between these two extreme states. For example, several of the patents and published applications referred to below describe electrophoretic displays in which the extreme states are white and deep blue, so that an intermediate “gray state” would actually be pale blue. Indeed, as already mentioned the transition between the two extreme states may not be a color change at all.
The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in published U.S. Patent Application No. 2002/0180687 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays.
The term “gamma voltage” is used herein to refer to external voltage references used by drivers to determine voltages to be applied to pixels of a display. It will be appreciated that a bistable electro-optic medium does not display the type of one-to-one correlation between applied voltage and optical state characteristic of liquid crystals, the use of the term “gamma voltage” herein is not precisely the same as with conventional liquid crystal displays, in which gamma voltages determine inflection points in the voltage level/output voltage curve.
The term “impulse” is used herein in its conventional meaning of the integral of voltage with respect to time. However, some bistable electro-optic media act as charge transducers, and with such media an alternative definition of impulse, namely the integral of current over time (which is equal to the total charge applied) may be used. The appropriate definition of impulse should be used, depending on whether the medium acts as a voltage-time impulse transducer or a charge impulse transducer.
Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type as described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791 (although this type of display is often referred to as a “rotating bichromal ball” display, the term “rotating bichromal member” is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed to applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface. This type of electro-optic medium is typically bistable.
Another type of electro-optic display uses an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example O'Regan, B., et al., Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. Pat. No. 6,301,038, International Application Publication No. WO 01/27690, and in U.S. Patent Application 2003/0214695. This type of medium is also typically bistable.
Another type of electro-optic display, which has been the subject of intense research and development for a number of years, is the particle-based electrophoretic display, in which a plurality of charged particles move through a suspending fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspending medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949; 6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545; 6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333; and 6,704,133; and U.S. Patent Applications Publication Nos. 2002/0019081; 2002/0021270; 2002/0053900; 2002/0060321; 2002/0063661; 2002/0063677; 2002/0090980; 2002/0106847; 2002/0113770; 2002/0130832; 2002/0131147; 2002/0145792; 2002/0171910; 2002/0180687; 2002/0180688; 2002/0185378; 2003/0011560; 2003/0011868; 2003/0020844; 2003/0025855; 2003/0034949; 2003/0038755; 2003/0053189; 2003/0096113; 2003/0102858; 2003/0132908; 2003/0137521; 2003/0137717; 2003/0151702; 2003/0189749; 2003/0214695; 2003/0214697; 2003/0222315; 2004/0008398; 2004/0012839; 2004/0014265; and 2004/0027327; and International Applications Publication Nos. WO 99/67678; WO 00/05704; WO 00/38000; WO 00/38001; WO00/36560; WO 00/67110; WO 00/67327; WO 01/07961; WO 01/08241; WO 03/092077; and WO 03/107,315.
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called “polymer-dispersed electrophoretic display” in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned 2002/0131147. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
A related type of electrophoretic display is a so-called “microcell electrophoretic display”. In a microcell electrophoretic display, the charged particles and the suspending fluid are not encapsulated within capsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, International Application Publication No. WO 02/01281, and U.S. Patent Application Publication No. 2002/0075556, both assigned to Sipix Imaging, Inc.
Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, the aforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat. Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.
The bistable or multi-stable behavior of particle-based electrophoretic displays, and other electro-optic displays displaying similar behavior, is in marked contrast to that of conventional liquid crystal (“LC”) displays. Twisted nematic liquid crystals act are not bi- or multi-stable but act as voltage transducers, so that applying a given electric field to a pixel of such a display produces a specific gray level at the pixel, regardless of the gray level previously present at the pixel. Furthermore, LC displays are only driven in one direction (from non-transmissive or “dark” to transmissive or “light”), the reverse transition from a lighter state to a darker one being effected by reducing or eliminating the electric field. Finally, the gray level of a pixel of an LC display is not sensitive to the polarity of the electric field, only to its magnitude, and indeed for technical reasons commercial LC displays usually reverse the polarity of the driving field at frequent intervals.
In contrast, bistable electro-optic displays act, to a first approximation, as impulse transducers, so that the final state of a pixel depends not only upon the electric field applied and the time for which this field is applied, but also upon the state of the pixel prior to the application of the electric field. Furthermore, it has now been found, at least in the case of many particle-based electro-optic displays, that the impulses necessary to change a given pixel through equal changes in gray level (as judged by eye or by standard optical instruments) are not necessarily constant, nor are they necessarily commutative. For example, consider a display in which each pixel can display gray levels of 0 (white), 1, 2 or 3 (black), beneficially spaced apart. (The spacing between the levels may be linear in percentage reflectance, as measured by eye or by instruments but other spacings may also be used. For example, the spacings may be linear in L* (where L* has the usual CIE definition:L*=116(R/R0)1/3−16,
where R is the reflectance and R0 is a standard reflectance value), or may be selected to provide a specific gamma; a gamma of 2.2 is often adopted for monitors, and where the present displays are be used as a replacement for a monitor, use of a similar gamma may be desirable.) It has been found that the impulse necessary to change the pixel from level 0 to level 1 (hereinafter for convenience referred to as a “0-1 transition”) is often not the same as that required for a 1-2 or 2-3 transition. Furthermore, the impulse needed for a 1-0 transition is not necessarily the same as the reverse of a 0-1 transition. In addition, some systems appear to display a “memory” effect, such that the impulse needed for (say) a 0-1 transition varies somewhat depending upon whether a particular pixel undergoes 0-0-1, 1-0-1 or 3-0-1 transitions. (Where, the notation “x-y-z”, where x, y, and z are all optical states 0, 1, 2, or 3 denotes a sequence of optical states visited sequentially in time, list from earlier to later.) Although these problems can be reduced or overcome by driving all pixels of the display to one of the extreme states for a substantial period before driving the required pixels to other states, the resultant “flash” of solid color is often unacceptable; for example, a reader of an electronic book may desire the text of the book to scroll down the screen, and may be distracted, or lose his place, if the display is required to flash solid black or white at frequent intervals. Furthermore, such flashing of the display increases its energy consumption and may reduce the working lifetime of the display. Finally, it has been found that, at least in some cases, the impulse required for a particular transition is affected by the temperature and the total operating time of the display, and by the time that a specific pixel has remained in a particular optical state prior to a given transition, and that compensating for these factors is desirable to secure accurate gray scale rendition.
In one aspect, this invention seeks to provide a method and a controller that can provide accurate gray levels in an electro-optic display without the need to flash solid color on the display at frequent intervals.
Furthermore, as will readily be apparent from the foregoing discussion, the drive requirements of bistable electro-optic media render unmodified drivers designed for driving active matrix liquid crystal displays (AMLCD's) unsuitable for use in bistable electro-optic media-based displays. However, such AMLCD drivers are readily available commercially, with large permissible voltage ranges and high pin-count packages, on an off-the-shelf basis, and are inexpensive, so that such AMLCD drives are attractive for drive bistable electro-optic displays, whereas similar drivers custom designed for bistable electro-optic media-based displays would be substantially more expensive, and would involve substantial design and production time. Accordingly, there are cost and development time advantages in modifying AMLCD drivers for use with bistable electro-optic displays, and this invention seeks to provide a method and modified driver which enables this to be done.
Also, as already noted, this invention relates to methods for driving electrophoretic displays which enable long-term DC-balancing of the driving impulses applied to the display. It has been found that encapsulated and other electrophoretic displays need to be driven with accurately DC-balanced waveforms (i.e., the integral of current against time for any particular pixel of the display should be held to zero over an extended period of operation of the display) to preserve image stability, maintain symmetrical switching characteristics, and provide the maximum useful working lifetime of the display. Conventional methods for maintaining precise DC-balance require precision-regulated power supplies, precision voltage-modulated drivers for gray scale, and crystal oscillators for timing, and the provision of these and similar components adds greatly to the cost of the display.
(Strictly speaking, DC balance should be measured “internally” having regard to the voltages experienced by the electro-optic medium itself. However, in practice it is impracticable to effect such internal measurements in an operating display which may contain hundreds of thousands of pixels, and in practice DC balance is measured using an “external” measurement, namely the voltages applied to the electrodes disposed on opposed sides of the electro-optic medium. Furthermore, there are two assumptions normally made when discussing DC balance. Firstly, it is assumed, normally with good reason, that the conductivity of the electro-optic medium is not a function of polarity, so that pulse length is an appropriate way to track DC balance, when a constant voltage is applied. Secondly, it is assumed that the conductivity of the electro-optic medium is proportional to the applied voltage, so that one can use impulse to track DC balance.)
Furthermore, even with the addition of such expensive components, true DC balance is still not obtained. Empirically it has been found that many electrophoretic media have asymmetric current/voltage (I/V curves); it is believed, although the invention is in no way limited by this belief, that these asymmetric curves are due to electrochemical voltage sources within the media. These asymmetric curves mean that the current when the medium is addressed to one extreme optical state (say black) is not the same as when the medium is addressed to the opposed extreme optical state (say white), even when the voltage is carefully controlled to be precisely the same in the two cases.
It has now been found that the extent of DC imbalance in an electrophoretic medium used in a display can be ascertained by measuring the open-circuit electrochemical potential (hereinafter for convenience called the “remnant voltage” of the medium. When the remnant voltage of a pixel is zero, it has been perfectly DC balanced. If its remnant voltage is positive, it has been DC unbalanced in the positive direction. If its remnant voltage is negative, it has been DC unbalanced in the negative direction. This invention uses remnant voltage data to maintain long-term DC-balancing of the display.
As described in more detail below, one aspect of the present invention relates to use of a so-called “look-up table” method for driving a bistable electro-optic display having a plurality of pixels, this method taking account of the initial and desired final state of each pixel, and to a device controller for use in this method. In preferred forms of this look-up table method, there are stored not only the initial gray level of each pixel but also one or more prior states of each pixel prior to the initial state thereof, and the output signal is generated dependent upon the one or more prior states and the initial gray level.
The output signal generated in such a look-up table method commonly defines a plurality of separate impulses. For example, FIGS. 11A and 11B below illustrate a so-called “sawtooth” driving scheme which is arranged so that once a given pixel has been driven from one extreme optical state (i.e., white or black) towards the opposed extreme optical state by a pulse of one polarity, the pixel may not receive a pulse of the opposed polarity until it has reached the aforesaid opposed extreme optical state. Depending upon the initial and final states for a given transition, this sawtooth driving scheme may require from one to three pulses alternating in polarity.
Furthermore, the individual pulses within these sequences may themselves be composites of sub-pulses, and some of these sub-pulses may apply zero voltage to a pixel. For example, Table 2 below illustrates a drive scheme in which white-going pulses are applied in odd-numbered frames and black-going pulses are applied in even-numbered frames. In this drive scheme, white-going transitions are driven only on the odd frames, black-going transitions are driven only on the even frames, and in any frame in which the pixel is not being driven, zero voltage is applied; the total impulse applied to any given pixel is controlled by pulse-width modulation, i.e., by the number of odd or even frames in a sequence for which a non-zero voltage is applied to the pixel. This drive scheme may be combined with that shown in FIGS. 11A and 11B to yield a drive scheme in which a given transition may require a large number of sub-pulses. In view of these complications, hereinafter the term “superframe” will be used to denote a sequence of successive display scan frames needed to effect all necessary gray level changes from an initial image to a final image. Typically, a display update is initiated only at the beginning of a superframe.
Finally, it should be noted that, in a look-up table method which stores at least one prior state of each pixel in addition to the initial state, the prior state(s) stored are not necessarily spaced one superframe apart in time, and the first prior state is not necessarily one superframe before the initial state, since in at least some electro-optic displays it has been found that it is the sequence of successive gray levels applied to a given pixel which is most important in determining the impulse needed to effect a given transition rather than the length of time for which the pixel is maintained in these successive gray levels. For example, consider a two-bit (four gray level) display which is updated once per second, i.e., the superframe length is one second, and in which the impulse applied is determined by the initial state, final state and one prior state. If a given pixel is held at gray level 3 for four superframes and then at gray level 1 for five superframes, in calculating the impulse needed to drive that pixel to a final state of gray level 2, it may be desirable to set the single prior state used for the calculation at gray level 3 (i.e., the immediately preceding gray level different from the initial gray level of 1) rather than 1, the actual gray level one superframe prior to the initial level. In other words, in this form of the look-up table method, the list of prior states is changed only when a change in gray level occurs, not at each superframe.
In practice, it has been found that the impulse needed to effect accurate transitions between gray levels in a bistable electro-optic display is affected by both prior gray state levels and gray state levels at specific times prior to the initial state, and in one aspect this invention provides a modified look-up table method and controller which allows adjustment of the impulse of a transition to allow for both types of parameters.
It must also be recognized that, as discussed in more detail below, depending upon the number of prior states stored, the look-up tables used in look-up table methods may become very large. To take an extreme example, consider a look-up table method for a 256 (28) gray level display using an algorithm that takes account of initial, final and two prior states. The necessary four-dimensional look-up table has 232 entries. If each entry requires (say) 64 bits (8 bytes), the total size of the look-up table would be approximately 32 Gbyte. While storing this amount of data poses no problems on a desktop computer, it may present problems in a portable device. In another aspect, this invention provides a method for driving a bistable electro-optic display which achieves results similar to those of the look-up table method but which does not require the storage of very large look-up tables.
A further aspect of the present invention relates to methods and apparatus for driving a bistable electro-optic display in a manner which permits part of the display to operate at a different bit depth (i.e., different number of gray scale levels) from the remainder of the display. From the foregoing description of the sawtooth driving method illustrated in FIGS. 11A and 11B below, it will be apparent to those skilled in the art that transitions between successive images in general image flow of bistable electro-optic displays having numerous gray scale levels can be substantially longer than transitions if the same displays were being driven in monochrome mode. Typically, gray scale transitions may be up to four times as long as the corresponding monochrome transitions. The relatively slow gray scale transitions may not be objectionable when the display is being used to present a series of images, such as a series of photographs or successive pages of an electronic book. However, there are times when it would be useful to achieve rapid updating of a limited area of such a display. For example, consider a situation where a user employs such a display to review of series of photographs stored in a database in order to enter for each photograph key words or other indexing terms intended to facilitate later retrieval of images from the database. In this situation, relatively slow transitions between successive photographs may be tolerable; for example, if the user spends one to two minutes studying each photograph and deciding on the indexing terms, a one to two second transition between successive photographs does not greatly affect the user's productivity. However, as is well known to anyone who has tried to run a word processing program on a computer with inadequate processing power, a one to two second delay in updating a dialog box, in which are displayed the indexing terms being entered by the user, is extremely frustrating and likely to lead to numerous typing errors. Accordingly, in this and similar situations, it would be advantageous to be able to run the dialog box in a monochrome mode to permit swift transitions, while continuing to run the remainder of the display in a gray scale mode to enable the images to be reproduced accurately, and this invention provides a method and apparatus to enable this to be done.
Another aspect of the present invention relates to methods to achieve fine control of gray levels of an impulse drive imaging medium without the need for fine voltage control. Although as already indicated, electrophoretic and some other electro-optic displays exhibit bistability, this bistability is not unlimited, and images on the display slowly fade with time, so that if an image is to be maintained for extended periods, the image may have to be refreshed periodically, so as to restore the image to the optical state which it has when first written.
However, such refreshing of the image may give rise to its own problems. As discussed in the aforementioned U.S. Pat. Nos. 6,531,997 and 6,504,524, problems may be encountered, and the working lifetime of a display reduced, if the method used to drive the display does not result in zero, or near zero, net time-averaged applied electric field across the electro-optic medium. A drive method which does result in zero net time-averaged applied electric field across the electro-optic medium is conveniently referred to a “direct current balanced” or “DC balanced”. If an image is to be maintained for extended periods by applying refreshing pulses, these pulses need to be of the same polarity as the addressing pulse originally used to drive the relevant pixel of the display to the optical state being maintained, which results in a DC imbalanced drive scheme.
A challenge for achieving accurate gray scale levels in an impulse driven medium is applying the appropriate voltage impulse for achieving the desired gray tone. Satisfactory transitions between optical states can be achieved by fine control of the voltage of all or part of the drive waveform. The need for precision can be understood from the following example. Consider the case where a current image consists of a screen that is half black and half white, and the desired next image is a uniform gray intermediate between black and white. In order to achieve a uniform gray level, the impulses used to go from black to gray and white to gray have to be finely adjusted so that the gray level achieved coming from black matches the gray level coming from white. Fine tuning is further needed if the final gray level achieved is a function of prior gray level history of the display. For example, as already discussed, the optical state achieved when going from black to gray can be a function, not only of the waveform applied, but also of what state was visited before the current black state. It is then desirable to have the display module keep track of some aspects of the display history, such as prior image states, and allow fine tuning of the waveform to compensate for this prior state history (see below for more detailed discussions on this point).
Fine tuning of the impulse can be achieved using only three voltage levels (0, +V, −V), by adjusting the width of the applied pulse with high accuracy. However, this is not desirable for an active matrix display, since the frame rate must be increased in order to achieve high pulse width resolution. A high frame rate increases the power consumption of the display, and puts more strenuous demands on the control and drive electronics. It is therefore not desirable to operate an active matrix display at frame rates substantially above 60-75 Hz.
Fine tuning of the impulse can also be achieved if a number of finely-spaced voltages are available. In an active matrix drive, this requires source drivers that can output one of a numerous set of voltages available over at least a subset of the available voltages. For example, for a driver that outputs between −10 and +10 volts, it may be advantageous to have available 0 V, and two bands of voltages between −10 and −7 volts and between 7 and 10 volts, with 16 distinct voltage levels between −10 and −7 volts and 16 distinct voltage levels between 7 and 10 volts bringing the total number of required voltage levels to 33 (see Table 1). One could then achieve fine control of the optical final state, for example, by varying the voltage between +7 and +10 or between −10 and −7 volts for the last one or more scan frames of the addressing period. This method is an example of a voltage-modulated technique for achieving acceptable display performance.
TABLE 1Example of voltages needed for voltagemodulated drive−10.0V−9.8V−9.6V−9.4V−9.2V−9.0V−8.8V−8.6V−8.4V−8.2V−8.0V−7.8V−7.6V−7.4V−7.2V−7.0V0.0V7.0V7.2V7.4V7.6V7.8V8.0V8.2V8.4V8.6V8.8V9.0V9.2V9.4V9.6V9.8V10.0V
The disadvantage of using voltage-modulated techniques is that drivers must have some range of fine voltage control. Display module cost can be reduced by using drivers that offer only two or three voltages.
In another aspect, this invention seeks to provide methods for achieving fine control of gray levels using drivers with only a small set of available voltages, specifically, where the control of impulse is too coarse to achieve the fine tuning necessary for acceptable display performance. Thus, this aspect of the present invention seeks to provide methods to achieve fine control of gray levels of an impulse driven imaging medium without the need for fine voltage control. This aspect of the invention can be applied, for example, to an active matrix display that has source drivers that can output only two or three voltages.
In another aspect, this invention relates to a method of driving an electro-optic display using a drive scheme that contains at least some direct current (DC) balanced transitions. For reasons explained at length in the aforementioned copending applications, when driving an electro-optic display it is desirable to use a drive scheme that is DC balanced, i.e., on which has the property that, for any sequence of optical states, the integral of the applied voltage is zero whenever the final optical state matches the initial optical state. This guarantees that the net DC imbalance experienced by the electro-optic layer is bounded by a known value. For example, a 15V, 300 ms pulse may be used to drive an electro-optic layer from the white to the black state. After this transition, the imaging layer has experienced 4.5 V-s of DC-imbalance impulse. To drive the film back to white, if a −15V, 300 ms pulse is used, then the imaging layer is DC balanced across the series of transitions from white to black and back to white.
It has also been found desirable to use a drive scheme in which at least some of the transitions are themselves DC balanced; such transitions are hereinafter termed “DC balanced transitions”. A DC-balanced transition has no net voltage impulse. A drive scheme waveform that employs only DC-balanced transitions leaves the electro-optic layer DC balanced after each transition. For example, a −15V, 300 ms pulse followed by a 15V, 300 ms pulse might be used to drive the electro-optic layer from white to black. The net voltage impulse across the electro-optic layer across this transition is zero. One might then use a 15V, 300 ms pulse followed by a −15V, 300 ms pulse to drive the electro-optic layer back to white. Again, the net voltage impulse is zero across this transition.
A drive scheme composed of all DC-balanced transition elements is, by necessity, a DC-balanced waveform. It is also possible to formulate a DC-balanced drive scheme that contains DC-balanced transitions and DC-imbalanced transitions, as discussed in detail below.